1. Algorithms
10 IST – Topic 6

2. What is an algorithm?
“a series of detailed instructions or steps that will solve a problem in a set amount of time”

3. Common examples of algorithms
Appliance Instructions
a logical set of steps to operate an appliance

4. Common examples of algorithms
Recipes
a logical set of steps to cook an item

5. Common examples of algorithms
Directions
a logical set of steps to arrive at a location

6. Common examples of algorithms
Repair Manual
a logical set of steps to repair an item

7. Advantages of using algorithms
solve problems in a logical step-by-step process avoiding vague “blundering-in-the-darkness”
finite – always has an end
methods are standardised
language is standardised
problems are described in the same way
terminology is uniform

8. Representing algorithms
Pseudocode
precise form of English that uses keywords and rules of structure
Flowcharts
a pictorial method of describing algorithms using a set of symbols, connecting lines and arrows.

9. Pseudocode Keyword Meaning BEGIN END to start a program
to finish a program
INITIALISATION
END INITIALISATION
to set any values at the start of a program
to end the values section
BEGIN SUBPROGRAM
END SUBPROGRAM
to start a subprogram
to finish a subprogram
IF <condition> THEN
ELSE
ENDIF
to allow selection or choice
for another choice (may not be needed)
to end the choice
CASEWHERE <condition> OTHERWISE
ENDCASE
to allow for many choices
to end the choices
WHILE <condition>
ENDWHILE
to begin a loop at the start of a sequence
REPEAT
UNTIL <condition>
to begin a loop at the end of a sequence

10. Pseudocode Rules Keywords are written in CAPITALS
Basic keywords come in pairs … for every BEGIN there is an END
Indenting is used to show structure
Names of subprograms are underlined

11. Flowcharts

12. Flowchart Rules
There is only ever ONE WAY into a flowchart structure and ONE WAY out.
A single flowchart should fit on one page
If larger than one page … use subprograms

13. Control Structures
there are 3 basic control structures to show the order in which statements are carried out:
sequencing (or steps)
selection (or choice)
repetition (or loops)

14. Sequencing most common
each step is carried out in order of its position
each step is done only once.

15. Selection Binary selection allows the choice between 2 possible paths
if one condition is met then one path is taken, otherwise the other path is taken

16. Selection Case or Multi-way selection
allows the choice between 3 or more possible paths
if the condition is met (TRUE) then one path is taken, otherwise one of the other paths is taken

17. Repetition
carries out a particular action any number of times until a condition is met
a loop is created to return the program to a point where the repetition starts and continues until the condition is met
the loop must have a terminating (ending) condition that is…
tested at some time during each repetition
updated during each repetition

18. Repetition Pre-test Repetition (While Loop / Guarded Loop)
the condition is tested at the start of the loop
if the condition is false the first time the processes will NEVER be carried out.
Pre-test loops end when the condition is false.
Uses WHILE … ENDWHILE

19. Repetition Post-test Repetition (Repeat-Until Loop / Unguarded Loop)
the condition is tested at the end of the loop after the loop has been run through
Post-test loops end when the condition is true
Uses REPEAT … UNTIL

20. Repetition For-Next Repetition
The body of the loop is executed a set number of times.
A counter variable is given a starting value which is incremented each time the loop is executed until a given final value is reached